Hyperon spin correlation in high-energy heavy-ion collisions

TL;DR

This work addresses the puzzle of large spin alignment signals observed in high-energy heavy-ion collisions by proposing that short-distance fluctuations of a vector strong-force field (SFF) induce spin correlations among nearby quarks and, consequently, among produced hyperons. Using a (3+1)D viscous hydrodynamic framework with fluctuating SFFs and a Gaussian space-time correlation, the authors compute Λ/Λ̄ spin correlations and introduce a net spin-correlation observable c_net^{ab} that exploits the opposite SFF response of ΛΛ/Λ̄Λ̄ versus ΛΛ̄ pairs to distinguish SFF effects from hydrodynamics. They find that the SFF-driven correlations yield opposite signs for same-sign and mixed-sign hyperon pairs, with a predicted magnitude of order 10^−4 for a correlation length around 0.6 fm, and a strong Δφ/ΔY dependence, suggesting a feasible experimental probe at RHIC BES-II. The results provide a concrete pathway to constrain short-range color-field fluctuations in the QGP and to separate non-perturbative strong-force effects from collective hydrodynamic dynamics in spin polarization phenomena.

Abstract

Recent experimental data show an unexpectedly large spin alignment of $φ$ mesons in high-energy heavy-ion collisions, which can be explained by short-distance fluctuations of strong-force fields (vector $φ$ fields) within the constituent-quark model. We calculate the hyperon spin correlations within the same model, taking into account hydrodynamic effects and a $φ$ field fluctuating in space-time according to a Gaussian distribution. The $Λ\barΛ$ spin correlation induced by the $φ$ field is shown to be negative as opposed to that of $ΛΛ$ or $\barΛ\barΛ$. We thus propose a new net spin-correlation observable as a sensitive probe to separate strong-force effects from hydrodynamic ones. With the strength of the field fluctuations extracted from the observed $φ$ spin alignment, we predict the collision-energy dependence of the hyperon spin correlations and also investigate the dependence of the net spin correlation on azimuthal-angle and rapidity difference.

Figures (7)

• Figure 1: The collision-energy dependence of the $\Lambda\Lambda$ spin correlation, compared to experimental data STAR:2022fan (red stars with error bars) for the $\phi$ meson global spin alignment $\delta\rho_{00}\equiv \rho_{00}-1/3$. The $xx$, $yy$, and $zz$ components of $c^{ab}_{\Lambda\Lambda}$ are denoted by the orange long-dashed line, the red short-dashed line, and the cyan solid line, respectively. The global spin alignment is rescaled by a factor $\alpha=0.01$ such that it has the same order of magnitude as the spin correlation.
• Figure 2: The spin correlation component $c_{12}^{yy}$ as a function of the azimuthal-angle difference between particles $1$ and $2$ in the range $|Y|<1$, $p_T\in(0.5,\,3)$ GeV/c. Correlations in the presence of SFF for $\Lambda\Lambda$, $\Lambda\bar{\Lambda}$, and $\bar{\Lambda}\bar{\Lambda}$ are denoted by orange long-dashed, red short-dashed, and cyan solid lines, respectively. The contribution from hydrodynamic fields to $c_{\Lambda\bar{\Lambda}}^{yy}$ is shown by the green dash-dotted line, while $c_{\Lambda\Lambda}^{yy}$ and $c_{\bar{\Lambda}\bar{\Lambda}}^{yy}$ in absence of SFF are nearly identical with $c_{\Lambda\bar{\Lambda}}^{yy}$ and thus are not shown in this figure. The lines are calculated with $\sigma_{t,x}=0.6$ fm, while the uncertainty bounds shown by the shaded areas are determined by $\sigma_{t,x}=0.3,\,0.9$ fm, respectively.
• Figure 3: Components of the net spin correlation $c^{ab}_\text{net}$ as functions of the azimuthal-angle difference in the range $|Y|<1$, $p_T\in(0.5,\,3)$ GeV/c for Au+Au collisions at $\sqrt{s_\text{NN}}=27$ GeV. Results with the SFF are shown by the orange, red, and cyan lines, while the other lines are results in absence of the SFF.
• Figure 4: Components of the net spin correlation $c^{ab}_\text{net}$ as functions of the rapidity difference. Computational conditions and notations are the same as in Fig. \ref{['fig:net-Spin-correlation-phi']}.
• Figure S1: The net spin correlation as a function of $\delta p_T\equiv|p_{T,1}-p_{T,2}|$. Computational conditions are the same as in Fig. \ref{['fig:net-Spin-correlation-phi']}.